Scanning head printers are printers wherein a printhead moves across a page one line at a time. For example, a printhead may scan from left to right or right to left across a page, printing as it advances.
A printhead is the part of a printer which causes an image to be imprinted on the page. Often, a printhead includes a vertical array of print elements which form dots on a page. Different images are printed by different print elements being fired at different times and in different locations. Each element is capable of printing one row of dots across the page.
If the vertical dimension of the image to be printed is less than the vertical height of the printhead, then the image can be printed in one printhead pass. An example of such an image includes 9- to 12-point text characters, when printed by a 1/6th-inch high printhead. However, images to be printed are often bigger than the printhead, especially when printing computer graphics. Accordingly, the printer may print part of the image in one pass of the printhead and the remaining part in subsequent passes.
When images are printed in more than one pass of the printhead, the problem of print misregistration may occur. Print misregistration is a misalignment of dots resulting in poor print quality. It is caused by printing two vertically adjacent dots in different passes of the printhead moving in opposite directions. Misregistration is caused by a backlash from the changed motion of the printhead.
By printing unidirectionally, print misregistration is avoided, but printer speed is limited. Accordingly, scanning head printers generally print small images such as text characters which are smaller than or equal to the height of the printhead bidirectionally, and larger text characters and graphics unidirectionally. Alternatively, to maintain print quality, all images may be printed unidirectionally.
In short, the throughput of a scanning head printer is generally decreased when large images or graphics are printed, because unidirectional printing is necessary to maintain print quality.
This invention increases the throughput of a scanning head printer by providing a step-control algorithm which, under proper circumstances, allows some graphic images to be printed bidirectionally.